1. Field Of The Invention
The present invention relates to the dispersal of a reaction process gas into a liquid carrier stream. More particularly, the present invention relates to the dispersal of chlorine in a paper pulp slurry for the purpose of pulp bleaching.
2. Prior Art
Essential to successful wood pulp bleaching is good dispersal of chlorine among the fibers of the pulp stock flow stream. On a weight ratio basis of elemental chlorine to oven dry pulp, only 4 to 6% chlorine is required. Although chlorine is generally considered to be highly soluble in water, the 4% to 6% chlorine required by the pulp in the residence time permitted by the bleach flow stream may exceed the solution capacity of the water vehicle which suspends the pulp as an aqueous slurry. In other words, a 3% solids consistency pulp slurry may comprise 1,000 lbs./min. of pulp dispersed in 30,500 lbs./min. of water. To bleach the 1,000 lbs./min. of pulp with 6% chlorine requires a chlorine flow rate of 60 lbs./min. However, for distribution to the 1,000 lbs./min. of pulp dispersed in 30,500 lbs./min. of water, 1.67 lbs. of chlorine per minute must be dissolved in a water volume flow rate of 100 gallons per minute (1.67 lbs. Cl.sub.2 /100 gal. H.sub.2 O).
At the temperatures and pressures normally used in this process, water will accept only 9.5 lbs. of chlorine per 100 gallons of water. This is an equilibrium ratio achieved after an extended period of time (hours) in agitated mixing. In a typical pulp mill bleach line, however, only 15 to 30 minutes are allotted to dissolve nearly 20% of the equilibrium capacity of the pulp slurry flow stream for chlorine. Such demands tend to press the solubility rate limits of the chemical system. For such reasons, mixers and other mechanical devices are often employed to assist the effort.
To compound the difficulties of achieving chlorine solution and uniform distribution within a pulp slurry flow stream is the physical circumstance that under standard conditions chlorine is a gas. Consequently, for economic reasons, the element is cooled and pressurized to the liquid state for large quantity transport. This necessitates heating and revaporization at the point of use to prevent explosive phase transfer when the element is blended with a pulp slurry.
In the normal course of preblending preparations, gaseous chlorine is combined with a liquid carrier flow stream of filtrate from chlorine and/or chlorine dioxide washers, for example. The liquid filtrate flow stream is then entrained with gaseous chlorine bubbles. The bubble entrained filtrate stream is subsequently blended with the primary stock flow stream.
The size and number of entrainment bubbles greatly influences the uniformity of a bleaching result. Obviously, more uniform distribution of the chlorine may be obtained from a large number of very small bubbles entrained in the carrier stream than a small number of relatively large bubbles, the absolute quantity of chlorine being the same in either case.
Although the bubble size of an entrainment apparatus such as a phase mixing injector may be controlled to a small degree by injector design, the most significant factor of bubble size is the pressure differential between the gas and liquid flow streams i.e. ##EQU1## where .DELTA.P is the pressure drop across the injector. As applied to a pulp stream chlorination unit, however, the pressure differential obtainable is limited by practical considerations.
For example, the static head and fluid friction losses in a chlorination system including the tower determine the backpressure for the chlorine-water injector which is usually in the order of 40 psig. Pressures typical to the chlorine supply system are in the order of 60 to 80 psig. A practical adjustment in either of these pressures would only marginally decrease the size of chlorine bubbles created by the injector. Even doubling of the .DELTA.P would only reduce the bubble size by 21%.
It is therefore, an object of the present invention to provide a method of dramatically decreasing the bubble size for gaseous chlorine added to a paper stock flow stream.
Another object of the invention is to accelerate the bleach reaction time for chlorine in a paper stock flow stream.
Another object of the present invention is to distribute bleaching chlorine more uniformly throughout a pulp stock flow stream.